1. Field of the Invention
The present invention relates to apparatus for disinfecting water by the direct introduction of gaseous ozone into the water. More particularly, the present invention relates to a water treatment apparatus in which gaseous ozone is introduced into a vertically disposed water conduit, such as by a plurality of porous diffuser elements positioned within the conduit or by direct injection.
2. Description of the Related Art
The notion of treating water with gaseous ozone for disinfection is well known. Various types of apparatus and methods have been devised to introduce and dissolve gaseous ozone into water for treatment of the water by providing retention chambers in which the ozone is introduced through mechanical mixers, injectors, porous diffusers, and packed columns. The primary purpose of each of those approaches is to transfer sufficient gaseous ozone to the water to be treated, in order to provide the required contact time of the ozone with the water so that the disinfection treatment is as thorough as possible.
Although there are several known approaches to accomplish the disinfection operation, even the more efficient of those processes, in terms of the efficiency of exposure of the water to ozone, include certain drawbacks. For example, although packed column technology provides transfer efficiencies of from about 80% to about 95%, there is little operational experience with such approaches, and, more significantly, there is a strong potential for high head losses as the result of build-up of scale on the column medium. Mechanical mixing, on the other hand, although capable of providing ozone transfer efficiencies in the range of from about 90% to about 98%, involves brief ozone-water contact times that might require retreatment for effective disinfection. Additionally, mechanical mixing also involves high operating cost because of the energy usage and the requirement for maintenance of the mechanical equipment.
Fine bubble diffuser contactors, another approach for effecting disinfection of water by the introduction of ozone, are quite widely used and provide ozone transfer efficiencies in the range of about 90% to about 95%. They operate effectively at low to medium ozone concentrations ( less than 6% by wt.) when there is a sufficient volumetric ozone flow rate delivered to the ozone contactor to effectively distribute the ozone to the diffusers and provide adequate ozone-water mixing for homogenous ozone dissolution. However, at higher ozone concentrations (8 to 12% by wt.) and lower gas flow rates, which often result when oxygen feed gas systems and advanced technology ozone generators are employed, there is an increased risk of gas bubble channeling, with resultant inadequate contact between the ozone gas bubbles and the water being treated, which could cause incomplete disinfection of the water.
To overcome inefficient mixing conditions in fine bubble diffuser systems at higher ozone concentrations, the present inventor developed an improved ozone dissolution system, combining a sidestream venturi injection system with a specially designed downflow tube to convey the ozone-water mixture into an ozone contactor. The downflow tube can be installed within or adjacent to one or more diffusion chambers of the ozone contactor to increase the time of contact of the ozone with the water. Turbulent mixing conditions are provided by the sidestream venturi injection system, even at low plant flows or low ozone doses. Pressurized sidestream flows discharge into the downflow tube through one or more venturi injectors, which also pull ozone gas onto the sidestream flows under negative pressure. In addition, the vertically oriented downflow tube maximizes bubble hold-up, thereby increasing bubble contact time with the water and improving ozone mass transfer rates. An example of such a system is disclosed in U.S. Pat. No. 5,273,664, which issued on Dec. 28, 1993, and is entitled, xe2x80x9cApparatus and Method for Disinfection of Water by Ozone Injection.xe2x80x9d That patent discloses the combination of sidestream injection and injection within the downflow tube to introduce ozone into the water. However, some venturi injection systems also exhibit incomplete mixing.
There have been recent advances in ozone generation technology by several equipment suppliers, and it is now possible to generate ozone at concentrations as high as 15% by weight. However, there still exists a need to develop a cost-effective ozone contacting device that is fully compatible with those equipment advances, and that can be used effectively for primary disinfection of drinking water. In particular, the inactivation of Cryptosporidium requires higher ozone concentrations and longer contact times than for the inactivation of other microorganisms of interest (e.g., Giardia and viruses). Consequently, careful attention to ozone mass transfer and homogeneous, two-phase mixing considerations in the ozone contactor are of critical importance.
It is an object of the present invention to provide a simplified and efficient apparatus for introducing low volumetric flows and high concentrations of ozone into water, for maintaining longer and more turbulent ozone-water contact in order to provide more complete disinfection of water, for providing positive mixing action at a point downstream of the ozone introduction point, and for positive sealing against backflow when the apparatus is out of service.
Briefly stated, in accordance with one aspect of the present invention, apparatus is provided for introducing ozone into water for treatment for disinfection purposes. The apparatus includes a contactor vessel for receiving water to be treated, the vessel including a water inlet conduit for introducing untreated water into the vessel and a water outlet conduit for withdrawing treated water from the vessel. At least one diffusion chamber is provided within the contactor vessel and within which ozone is introduced into water to be treated, and at least one downflow tube having a longitudinal axis is positioned substantially vertically within the diffusion chamber. The downflow tube includes a water inlet and a water outlet, wherein the water outlet is positioned below the water inlet. An ozone distributor is positioned within the downflow tube between the downflow tube water inlet and the downflow tube water outlet. The ozone distributor is in communication with a source of ozone for introducing the ozone into the downflow tube. A check valve is positioned at the downflow tube water outlet for closing the outlet opening and preventing backflow of water into the outlet opening upon cessation for flow of untreated water into the downflow tube inlet.